The invention disclosed herein involves the processing of multiple channels of electrical signals produced by the heart. These channel signals include the signals from electrodes within the body, i.e., intracardiac signals from within vessels and chambers of the heart and epicardial signals from the outer surface of the heart. Throughout this document, the term “multi-channel cardiac electrogram” (or “MCCE”) is used to refer to all of these types of channels; when specific types are appropriate, specific nomenclature is used. This terminology (MCCE) is used herein since the term “ECG” sometimes only refers to body-surface measurements of cardiac performance.
A major component in cardiac interventional procedures such as cardiac ablation is the display of cardiac data extracted from the MCCE signals captured by an array of electrodes placed within and on the structures of the heart itself. Among the important data displayed are intracardiac cycle length (time between the activations in arrhythmias (such as atrial fibrillation), relative time differences between related activations in two intracardiac channels to generate activation maps, and assessments of signal strength, variability and other measures of signal quality within MCCE signals.
Cardiac interventional electrophysiology procedures (e.g., ablation) can be extremely time-consuming, and the reliable determination and presentation of such cardiac parameters is an important element in both the quality of the procedures and the speed with which they can be carried out. Often the data presented to the electrophysiology doctor during such procedures exhibit high variability contributed not only by the performance of the heart itself but by unreliable detection of certain features of the MCCE signals. Therefore, there is a need for more reliable and more rapid algorithms to process intracardiac signals obtained during an electrophysiology (EP) procedure.
MCCE electrodes capture the electrical signals in the cardiac muscle cells. As mentioned above, some MCCE electrodes may be positioned inside cardiac veins, arteries and chambers (intracardiac) and on the outer surface of the heart (epicardial) as conductive elements at the tips or along the lengths of catheters introduced into the body and maneuvered into position by the EP doctor. The electrical signals within the heart muscles and which flow therefrom to other regions of the body have very low voltage amplitudes and therefore are susceptible to both external signal noise and internally-generated electrical variations (non-cardiac activity). In addition, cardiac arrhythmias themselves may be highly variable, which can make reliable extraction of cardiac parameters from MCCE signals difficult.
One important cardiac parameter used during such procedures is the time difference between the activations occurring within two channels, both of which contain the electrical signals of an arrhythmia. This measurement is called local activation time (LAT), and measurement of a plurality of values of LAT is the basis for the generation of an activation map. The map displays information about the sequence of activations of cardiac muscle cells relative to each other, and this sequence of information is combined with physical anatomical position information to form the map. An activation map then provides guidance to the EP doctor for the process of applying therapies to heart muscle cells which can terminate cardiac arrhythmias and permanently affect the heart to prevent recurrence of arrhythmias.
The entire process of determining LAT is referred to as mapping because all of the information generated by analysis of the MCCE signals is combined in a single computer display of a three-dimensional figure that has the shape of the heart chamber of interest and employs additional image qualities such as color which convey the sequence of electrical activity (activation map) or possibly other qualities of the electrical activity (e.g., voltage map). These images are similar in style to weather maps common today in weather-forecasting. Such a cardiac map becomes a focus of attention for the EP doctor as he directs the motion of catheters in the heart to new positions, and an algorithm which processes the MCCE signals produces measurements from the electrodes in new positions. As this process continues, the map is updated with new colored points to represent additional information about the electrical activity of the heart.
During a mapping procedure, the timing relationships of muscle depolarizations typically must be determined for hundreds of locations around a heart chamber which may be experiencing an abnormal rhythm. The locations are often examined, one at a time, by moving an exploring cardiac-catheter electrode (mapping-channel electrode) from location to location, acquiring perhaps only a few seconds of signal data at each location. To compare timing relationships, a different electrode (reference-channel electrode) remains stationary (at a single location) and continuously acquires a reference signal of the rhythm.
The collection of timing relationships and anatomical locations constitutes an activation map (LAT map). As described above, a relatively large number of individual LAT values are used to generate a useful LAT map. Many different locations may serve adequately as alternative reference locations, but it has been critical in the present state-of-the-art that whatever location is used as the reference, one activation map is committed for the entire duration to that reference location only.
U.S. Pat. No. 8,812,091 (Brodnick), titled “Multi-channel Cardiac Measurements” and filed on Jun. 20, 2013, discloses several aspects of improved methods for determining LAT. (Such patent and the invention of the present application are commonly-owned, and Donald Brodnick is also an inventor of the present invention.) The Brodnick patent discloses LAT-determination methods which include replacement of cardiac channels when the quality of such channel signals falls below a standard measure of channel-signal quality. Major portions of the disclosure of the Brodnick patent is included herein since it provides excellent background information for the improved LAT-determination methods disclosed herein.
Occasionally a reference electrode is bumped or becomes disconnected. In these cases, additional data cannot be collected to extend the map (add more LAT values to the map) because the timing relationships are no longer comparable (based on the same reference-channel signal). The EP doctor either makes his or her interpretation of the map based on an incomplete map or establishes a new reference and begins to create a new map, having lost the time and effort which to this point in the procedure had been expended. At a few seconds of signal acquisition per location, a few seconds of catheter motion between locations, and hundreds of locations, the amount of time and effort wasted if a map must be restarted can be very significant. Furthermore, extending the total procedure time adds more risk of complications for the patient.
Because the heart is constantly contracting and other catheters are continually being repositioned, a procedure may last for several hours, during which time the patient even may need to be moved. Occasionally a reference electrode either makes poor contact or may shift position, in which case the constant timing relationship is disrupted (timing stability is lost) and additional locations cannot be studied in relationship to the accumulated data. As described above, the resulting incomplete activation map may be worthless, requiring a new map, extending the procedure and adding cost and risk to the patient.
Thus there is a need for an automatic method of determining local activation time (LAT) from multi-channel cardiac electrogram signals which avoids substantial loss of LAT values in spite of losses of timing stability in reference channels during a local activation time mapping procedure.
The generation of position information and its combination with cardiac timing information is outside the scope of the present invention. The focus of the present invention is the processing of MCCE signals to measure time relationships within the signals, the two most important of which are cycle length (CL) and local activation time (LAT).
Currently-available MCCE-processing algorithms are simplistic and often provide inaccurate measurements which cause the activation map and many other cardiac parameter values to be misleading. A misleading map may either (1) compel the EP doctor to continue mapping new points until apparent inconsistencies of the map are corrected by a preponderance of new, more-accurately measured map points or (2) convince the EP doctor to apply a therapy to a muscle region which actually makes little or no progress in the termination of an arrhythmia, again prolonging the procedure while more points are mapped in an attempt to locate new regions where therapy may be effective.
Currently, computer systems which assist EP doctors in the mapping process have manual overrides to allow a technician, or sometimes the EP doctor himself, to correct the measurements made automatically by the system. This requires a person to observe a computer display presentation called the “Annotation Window” which shows a short length of the patient's heart rhythm, perhaps 3-5 heartbeats as recorded in 3-8 channels (signals from MCCE electrodes).
The channels of the annotation window are of several types. There is one channel, identified as a reference channel, the electrode of which ideally remains in a fixed position during the entire map-generating procedure, and there is at least one other intracardiac channel (the mapping channel) which senses the electrical signal at a catheter tip, the precise three-dimensional position of which is determined by other means. The electrical activity in the mapping channel is compared to the activity in the reference channel to determine the local activation time (LAT) which is used to color the map at that precise three-dimensional position.
Intracardiac channels may be of either the bipolar or unipolar recording types, and the inventive measurement method disclosed herein can be applied to both types of signals. Also, since it is possible during arrhythmias for some chambers of the heart to be beating in a rhythm different from other chambers of the heart, the annotation window often contains additional channels to aid in the interpretation of the data presented.